This invention relates to an integrated circuit that with a stable capacitor attached becomes a stable oscillator, and more particularly pertains to such an oscillator wherein the capacitor charging and discharging currents are each proportional to a reference current from a current source.
Such an oscillator is described in the patent application to Gontowski Ser. No. 296,749 filed Aug. 27, 1981 that matured into the U.S. Pat. No. 4,450,415 on May 22, 1984 and is assigned to the same assignee as is the present application. A reference current source is provided in the integrated circuit. The reference current is switched back and forth between two differentially connected transistors to, respectively, charge and discharge the capacitor. The bases of the two differentially connected transistors are, respectively, connected to the capacitor and an intermediate voltage point in a voltage divider.
The capacitor charge interval terminates when the capacitor voltage reaches the intermediate voltage. During the subsequent discharge interval, the voltage at the voltage divider intermediate point is reduced by activating a transistor to shunt part of the divider. The discharge interval terminates when the capacitor voltage reaches the above noted reduced voltage.
That reduced voltage is dependent upon V.sub.(SAT) of the shunting transistor. When it is desired to operate the oscillator at a low supply voltage, the shunt-transistor V.sub.(SAT) becomes an especially significant factor in the determination of the peak-to-peak voltage across the oscillating capacitor and thus a more significant factor determining the frequency of oscillation. Such transistor saturation voltages collector-to-emitter, V.sub.(SAT), vary from piece to piece due to variations in integrated circuit (IC) processing and vary as a function of temperature (typically +0.1 MV/.degree.C.) and collector current which in turn is a function of DC supply voltage.
It is an object of the present invention to provide an oscillator, capable of being formed in an integrated circuit except for an external capacitor, providing a more stable operation with temperature and supply voltage, and providing more uniform performance piece to piece at the end of the IC production line.